1. Field of the Invention
The present invention relates to an air bearing structure and a linear driving device which is integrally formed with the air bearing structure.
2. Description of the Prior Art
In devices such as super precision processing devices which require positioning precision in nanometer order, an air bearing is used in order to eliminate friction during movement of the movable portions. Using the air bearing allows very high precision operations to be realized. However because the air is in the form of compressed liquid, the bearing rigidity deteriorates by one digit or more compared to that of the oil bearing or the rotor bearing.
In an air bearing, compressed air is supplied between bearing surface at the fixed portion side and the bearing surface at the movable portion side so that the movable part is supported by the pressure of this air. In the case of the air bearing in particular, in which the movable portion moves in a linear direction with respect to the fixed portion, the area of bearing surface on the side of the fixed portion, which opposes the bearing surface on the side of the movable portion, changes in accordance with the movement in the linear direction of the movable portion. As a result, if an air discharge opening such as a nozzle or the like is provided on the side of the fixed portion and air is supplied from the fixed portion side, the air from the air discharge opening that is formed in the area of the bearing surface on the side of the fixed portion which does not oppose the bearing surface on the side of the movable portion, is released into the atmosphere and the air pressure between the bearing surfaces is reduced. As a result in the prior art, in the case of the air bearing in which the movable portion moves in the linear direction with respect to the fixed portion, air supply is carried out from the movable portion side.
The prior art example of an air bearing in which air is supplied from the movable portion side and the movable portion moves linearly with respect to the fixed portion is described in the following using FIG. 2A to FIG. 2E.
As shown in FIG. 2B, the fixed portion 11 has a substantially square pillar shape, and the bottom surface thereof is fixed to the surface of the base 13, and extends upward from the surface of the base 13. All of the 4 surfaces of the fixed portion 11 have vertical grooves 11a, 11b, 11c and 11d which have a predetermined width, in the center portion in the width direction, from the bottom area of the fixed portion 11 to the top portion thereof. As a result, at the four corners of the four square pillars of the fixed portion 11, the portion excluding the vertical grooves 11a, 11b, 11c and 11d form protruding portions 11ab, 11bc, 11cd and 11da which protrude outward from the corners of the square pillar. In addition, an air bearing surface is formed at each of the 2 adjoining wall surfaces of the protruding portions 11ab, 11bc, 11cd and 11da. 
Meanwhile, a magnet 16 comprising a linear motor is mounted at the bottom surface of one vertical groove 11a and another opposing vertical groove 11c of the fixed portion 11. Furthermore, a scale 18 which is a linear position detector is mounted on the bottom surface of another vertical groove 11d, respectively.
On the other hand, as shown in FIG. 2C, a through-hole with a substantially square cross-section through which the fixed portion 11 can be inserted is formed in the movable portion 12. On the two opposing wall surfaces from among the four wall surfaces inside the movable portion 12 which surround the through hole, vertical grooves 12a and 12c having a predetermined width are formed at the center portion thereof in the width direction to extend from the bottom of the movable portion 12 to the top thereof. A coil 15 constituting a linear motor is mounted on the bottom surface of the vertical grooves 12a and 12c so that the coil 15 opposes magnet 16 which is mounted on the fixed portion 11. Furthermore, a detection head 17 is mounted on one wall surface which does not have vertical groove formed inside the movable portion 12 so that the detection head 17 opposes the scale 18 mounted on the fixed portion 11.
Furthermore, at the four corners where a wall surfaces meets another wall surface inside the movable portion 12a, a plurality of air discharge openings 14 such as nozzles through which air is injected toward the inside are formed in the vertical direction.
Thus, as shown in FIG. 2D, the fixed portion 11 is inserted into the through-hole of the movable portion 12 so that the magnet 16 of the fixed portion 11 and the coil 15 of the movable portion 12 oppose each other and the scale 18 of the fixed portion 11 and the detection head 17 of the movable portion 12 oppose each other. In this state, each of the two adjacent surfaces in the projecting portions 11ab, 11bc, 11cd and 11da at the four corners of fixed portion 11 oppose the surface of the movable portion 12 on which the air discharge openings are provided, with the result that these opposing surfaces constitute air bearing surfaces F, thereby forming an air bearing.
As shown in FIG. 2E, the vertical length Hs of the air bearing surface on the side of the fixed portion 11, which corresponds to the vertical length of the fixed portion 11, is longer than the vertical length Hm of the air bearing surface on the side of the movable portion 12, which corresponds to the vertical length of the movable portion 12. As shown in FIG. 2A, the movable portion 12 is connected with an air supply tube for supplying air to the air discharge openings 14, a power line for supplying power to the coil 15, and a cable 19 for signal lines and the like to the detection head 17.
By supplying compressed air using the cable 19 and injecting air onto the air bearing surface F using the air discharge openings 14 of the movable portion 12, the movable portion 12 is supported through air between the movable portion 12 and the fixed portion 11 without coming into contact with the fixed portion 11. In addition, if power is supplied to the coil 15 via the cable 19 to drive the linear motor composed of the magnet 16 on the fixed portion 11 and the coil 15 on the movable portion 12, the movable portion 12 is guided by the air bearing surface F of the fixed portion 11, and thus driven vertically in FIG. 2A. The movement amount of the movable portion 12 with respect to the fixed portion 11 is detected by a linear position detector comprising the scale 18 on the fixed portion 11 and the detection head 17 on the movable portion 12.
As shown above, in the linear drive device using the air bearing of the prior art, because the air discharge openings 14 are provided on the side of the movable portion 12, the supply tube for supplying pressurized air must be connected to the movable portion 12. Furthermore, as described above, cables for a power line and a signal line are also connected to the movable portion.
However, as described above, because in the air bearing, the air is a compressed liquid, the rigidity of the bearing is low. As a result, in prior art, each of cables that are attached to the movable portion 12 is disposed such that curvature of the cable is large enough to prevent a load on the cables. However, the effect of the load due to the cables and the like that are attached to the movable portion can not be disregarded, as the air bearing is influenced in nanometer order. Furthermore, because the air bearing surface receives a larger force due to the compressed air, the effect on accuracy of the deformation on mechanical structure cannot be disregarded.